This study of 164 married couples examined longitudinal links between parents' perceptions of coparenting support and undermining by spouse at 6 months postpartum and infant behaviour problems at the age of 12 months after controlling for marital quality, individual parenting, and infant temperament. Multiple methods (i.e. parent reports and direct observations) and sources (i.e. mothers and fathers) were used to assess marital, parenting, and coparenting dynamics. Higher levels of maternal undermining at the age of 6 months as reported by fathers were linked to greater infant behaviour problems at the age of 12 months. In addition, higher paternal reports of maternal coparenting support at the age of 6 months were also linked to greater infant behaviour problems at the age of 12 months. No results emerged for paternal undermining or paternal coparenting support as reported by mothers in predicting infant behaviour problems. These novel findings utilising parents' reports of coparenting dynamics suggest reports from fathers about their wives' coparenting behaviour are a useful supplement to information about the marriage, individual parenting, and infant temperamental characteristics in understanding infant behaviour problems. (Contains 3 tables.)